Atomic layer etching processes

ABSTRACT

Processing methods may be performed to remove unwanted materials from a substrate. The methods may include forming a remote plasma of an inert precursor in a remote plasma region of a processing chamber. The methods may include forming a bias plasma of the inert precursor within a processing region of the processing chamber. The methods may include modifying a surface of an exposed material on a semiconductor substrate within the processing region of the processing chamber with plasma effluents of the inert precursor. The methods may include extinguishing the bias plasma while maintaining the remote plasma. The methods may include adding an etchant precursor to the remote plasma region to produce etchant plasma effluents. The methods may include flowing the etchant plasma effluents to the processing region of the processing chamber. The methods may also include removing the modified surface of the exposed material from the semiconductor substrate.

TECHNICAL FIELD

The present technology relates to semiconductor systems, processes, andequipment. More specifically, the present technology relates to systemsand methods for removing material layers on a wafer surface.

BACKGROUND

Integrated circuits are made possible by processes which produceintricately patterned material layers on substrate surfaces. Producingpatterned material on a substrate requires controlled methods forremoval of exposed material. Chemical etching is used for a variety ofpurposes including transferring a pattern in photoresist into underlyinglayers, thinning layers, or thinning lateral dimensions of featuresalready present on the surface. Often it is desirable to have an etchprocess that etches one material faster than another facilitating, forexample, a pattern transfer process. Such an etch process is said to beselective to the first material. As a result of the diversity ofmaterials, circuits, and processes, etch processes have been developedwith a selectivity towards a variety of materials.

Etch processes may be termed wet or dry based on the materials used inthe process. A wet HF etch preferentially removes silicon oxide overother dielectrics and materials. However, wet processes may havedifficulty penetrating some constrained trenches and also may sometimesdeform the remaining material. Dry etches produced in local plasmasformed within the substrate processing region can penetrate moreconstrained trenches and exhibit less deformation of delicate remainingstructures. However, local plasmas may damage the substrate through theproduction of electric arcs as they discharge.

Thus, there is a need for improved systems and methods that can be usedto produce high quality devices and structures. These and other needsare addressed by the present technology.

SUMMARY

Processing methods may be performed to remove unwanted materials from asubstrate. The methods may include forming a remote plasma of an inertprecursor in a remote plasma region of a processing chamber. The methodsmay include forming a bias plasma of the inert precursor within aprocessing region of the processing chamber. The methods may includemodifying a surface of an exposed material on a semiconductor substratewithin the processing region of the processing chamber with plasmaeffluents of the inert precursor. The methods may include extinguishingthe bias plasma while maintaining the remote plasma. The methods mayinclude adding an etchant precursor to the remote plasma region toproduce etchant plasma effluents. The methods may include flowing theetchant plasma effluents to the processing region of the processingchamber. The methods may also include removing the modified surface ofthe exposed material from the semiconductor substrate.

In some embodiments, the inert precursor may include hydrogen or helium.The remote plasma region may include a region within the processingchamber separated from the processing region by a showerhead. The remoteplasma region may be characterized by a smaller gap between electrodeswithin the processing chamber than the processing region. The methodsmay be performed at a chamber operating pressure below about 500 mTorr.The etchant precursor may include a fluorine-containing precursor or anitrogen-containing precursor. The remote plasma may include acapacitively-coupled plasma. The remote plasma may be formed at anelectrical frequency of greater than or about 40 MHz. The remote plasmamay be formed at an electrical frequency less than or about 80 MHz. Themethods may further include halting a flow of the etchant precursorwhile maintaining the remote plasma. The methods may also includeforming a bias plasma in the processing region. The methods may alsoinclude modifying an additional amount of the exposed material. Theoperations of flowing and halting the flow of the etchant precursor maybe performed for a plurality of cycles. The remote plasma may bemaintained throughout the plurality of cycles.

The present technology may also encompass additional etching methods.The methods may include forming a first plasma within a remote plasmaregion of a processing chamber. The methods may include forming a secondplasma within a processing region of the processing chamber. The methodsmay include modifying an exposed material on a semiconductor substratewithin the processing region of the processing chamber with effluents ofthe second plasma. The methods may include extinguishing the secondplasma while maintaining the first plasma. The methods may includeproviding an etchant precursor to the remote plasma region to formetchant plasma effluents. The methods may include etching the modifiedexposed material on the semiconductor substrate.

In some embodiments, the etching may be performed at a temperature ofabout 100° C. The remote plasma region of the processing chamber may befluidly coupled with, and physically separated from, the processingregion of the processing chamber. The first plasma may be acapacitively-coupled plasma operated at a power level of about 500 W orless. The first plasma may be a capacitively-coupled plasma operated ata frequency of about 40 MHz or more. The methods may also includehalting a flow of the etchant precursor while maintaining the firstplasma. The methods may include reforming the second plasma in theprocessing region of the processing chamber. The methods may includemodifying an additional amount of the exposed material on thesemiconductor substrate.

The present technology may also encompass additional etching methods.The methods may include striking a plasma of an inert precursor in aremote plasma region of a processing chamber. The remote plasma regionmay be characterized by a first gap between electrodes within theprocessing chamber. The methods may include striking a plasma of theinert precursor within a processing region of the processing chamber.The processing region of the processing chamber may be characterized bya second gap between electrodes within the processing chamber. Thesecond gap between electrodes may be greater than the first gap betweenelectrodes. The methods may also include modifying a surface of asemiconductor substrate within the processing region of the processingchamber with plasma effluents of the inert precursor. The methods mayinclude extinguishing the plasma within the processing region of theprocessing chamber while maintaining the plasma in the remote plasmaregion. The methods may include flowing an etchant precursor to theremote plasma region to produce etchant plasma effluents. The methodsmay include flowing the etchant plasma effluents to the processingregion of the processing chamber. The methods may also include removingthe modified surface of the semiconductor substrate. In someembodiments, the plasma formed in the remote plasma region may be formedat a frequency of at least about 40 MHz.

Such technology may provide numerous benefits over conventional systemsand techniques. For example, by maintaining a plasma in a remote region,pressure swings within the chamber region may be limited. Additionally,the higher frequency of plasma generation may allow the remote plasma tobe formed at lower pressure. These and other embodiments, along withmany of their advantages and features, are described in more detail inconjunction with the below description and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the disclosedtechnology may be realized by reference to the remaining portions of thespecification and the drawings.

FIG. 1 shows a top plan view of an exemplary processing system accordingto the present technology.

FIG. 2 shows a schematic cross-sectional view of an exemplary processingchamber according to the present technology.

FIG. 3 shows selected operations in an etching method according toembodiments of the present technology.

FIGS. 4A-4B illustrate schematic cross-sectional views of substrateprocessing chambers in which selected operations may be performedaccording to embodiments of the present technology.

Several of the figures are included as schematics. It is to beunderstood that the figures are for illustrative purposes, and are notto be considered of scale unless specifically stated to be of scale.Additionally, as schematics, the figures are provided to aidcomprehension and may not include all aspects or information compared torealistic representations, and may include superfluous or exaggeratedmaterial for illustrative purposes.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a letter thatdistinguishes among the similar components. If only the first referencelabel is used in the specification, the description is applicable to anyone of the similar components having the same first reference labelirrespective of the letter.

DETAILED DESCRIPTION

The present technology includes systems and components for semiconductorprocessing of small pitch features. As line pitch is reduced, standardlithography processes may be limited, and alternative mechanisms may beused in patterning. Conventional technologies have struggled with theseminimal patterning and removal operations, especially when exposedmaterials on a substrate may include many different features andmaterials, some to be etched and some to be maintained.

Atomic layer etching is a process that utilizes a multiple-operationprocess of damaging or modifying a material surface followed by anetching operation. The etching operation may be performed at chamberconditions allowing the modified material to be removed, but limitinginteraction with unmodified materials. This process may then be cycledany number of times to etch additional materials. Some chambersavailable can perform both operations within a single chamber. Themodification may be performed with a bombardment operation at thesubstrate level, followed by a remote plasma operation to enhanceetchant precursors capable of removing only the modified materials.

Because of the volumetric distribution within a chamber between regionsformed between various components, one plasma region may be of greatervolume than another, or the distance between the electrodes may belarger in one region than in another. For example, the distance betweenelectrodes in the wafer-level plasma region may be greater than thedistance between electrodes in the remote plasma region. This may affectthe requirements for plasma formation within the regions. For example,according to the Paschen curve of various gases, the pressure within asystem and gap between the electrodes may impact the breakdown voltage.When the electrode gap is fixed, as in a particular processing chamber,the pressure may be increased to generate plasma in a region definedbetween closer electrodes, and lowered when generating plasma in aregion defined between electrodes farther apart.

Accordingly, continuing the example within the noted chamber, thepressure may be cycled back and forth between a lower pressure duringthe modification operation, and a higher pressure during the etchingoperation. Of course, this is only an example, and different chamberconfigurations may be characterized by the opposite pressure swing basedon the chamber design. The pressure swing may be fairly dramatic in someconditions. For example, the modification operation may be performed ata pressure below 1 Torr, including below or about 100 mTorr, or below orabout 50 mTorr. The etching operation may be performed at much higherpressure, such as above 1 Torr, including above or about 2 Torr, aboveor about 3 Torr, or more. Modulating the pressure between, say, 50 mTorrand 3 Torr may be a fairly drastic change which can cause multipleissues for the overall process.

A first issue may relate to queue times, or the time to process wafers.Although the pressure cycling may be performed in a relatively shorttime, such as between about 30 seconds or 1 minute, when the pressure ismodulated twice in each cycle of the operation, if a process isperformed for 10, 20, 30, 50 or more cycles, the increase in processingtime due to pressure cycling can be hours. Additionally, the significantdifference between the two pressure conditions may cause byproduct flowissues within a chamber. In some chambers materials removed from asubstrate are purged from the chamber. The path of purge, such as belowthe substrate support, may collect an amount of particulate material oretching byproducts, which normally may not interfere with the processedsubstrate. However, when the pressure is cycled between the twooperation points, the flow profile within the processing chamber maycause backflow of fluid, which may dislodge byproducts or otherparticulate materials, and deliver them back into the processing region,causing deposition on the substrate. These byproducts can cause manyknown problems, such as short circuiting of formed devices, or blockingof feature openings limiting later processing operations.

The present technology overcomes these issues by forming the remoteplasma at higher frequency, which may allow the plasma to be generatedat lower pressure. Accordingly, the pressure may be maintained betweenthe two operations, reducing queue times of the etching processes.Additionally, in some embodiments the remote plasma may be maintainedthroughout the entire process while cycling only the substrate-levelplasma. This may provide added benefits of limiting plasma striking inthe remote region. For example, when striking a plasma, a higher voltagemay be needed at initiation. This initial higher voltage can causeissues such as arcing, surface damage of chamber components, orsputtering of chamber surfaces. Once the plasma has been formed,however, maintaining the plasma may use lower voltage, which may limitthese effects. Accordingly, by maintaining the remote plasma throughoutthe processes of some embodiments of the present technology, theparticle generation and damage to chamber components may be reduced.Although a modification of the chamber may be performed, such as toincrease the gap in the remote plasma region, such a modification to thechamber may cause additional issues. For example, as the gap increases,plasma characteristics may change, and affect the etchant produced. Thismay cause a reduction in selectivity or other etchant characteristics.Hence, the present technology may maintain a gap distribution, and mayincrease the frequency of the remote plasma generation.

Although the remaining disclosure will routinely identify specificetching processes utilizing the disclosed technology, it will be readilyunderstood that the systems and methods are equally applicable todeposition and cleaning processes as may occur in the describedchambers. Accordingly, the technology should not be considered to be solimited as for use with etching processes alone.

FIG. 1 shows a top plan view of one embodiment of a processing system100 of deposition, etching, baking, and curing chambers according toembodiments. The processing tool 100 depicted in FIG. 1 may contain aplurality of process chambers, 114A-D, a transfer chamber 110, a servicechamber 116, an integrated metrology chamber 117, and a pair of loadlock chambers 106A-B. The process chambers may include structures orcomponents similar to those described in relation to FIG. 2, as well asadditional processing chambers.

To transport substrates among the chambers, the transfer chamber 110 maycontain a robotic transport mechanism 113. The transport mechanism 113may have a pair of substrate transport blades 113A attached to thedistal ends of extendible arms 113B, respectively. The blades 113A maybe used for carrying individual substrates to and from the processchambers. In operation, one of the substrate transport blades such asblade 113A of the transport mechanism 113 may retrieve a substrate Wfrom one of the load lock chambers such as chambers 106A-B and carrysubstrate W to a first stage of processing, for example, an etchingprocess as described below in chambers 114A-D. If the chamber isoccupied, the robot may wait until the processing is complete and thenremove the processed substrate from the chamber with one blade 113A andmay insert a new substrate with a second blade (not shown). Once thesubstrate is processed, it may then be moved to a second stage ofprocessing. For each move, the transport mechanism 113 generally mayhave one blade carrying a substrate and one blade empty to execute asubstrate exchange. The transport mechanism 113 may wait at each chamberuntil an exchange can be accomplished.

Once processing is complete within the process chambers, the transportmechanism 113 may move the substrate W from the last process chamber andtransport the substrate W to a cassette within the load lock chambers106A-B. From the load lock chambers 106A-B, the substrate may move intoa factory interface 104. The factory interface 104 generally may operateto transfer substrates between pod loaders 105A-D in an atmosphericpressure clean environment and the load lock chambers 106A-B. The cleanenvironment in factory interface 104 may be generally provided throughair filtration processes, such as HEPA filtration, for example. Factoryinterface 104 may also include a substrate orienter/aligner (not shown)that may be used to properly align the substrates prior to processing.At least one substrate robot, such as robots 108A-B, may be positionedin factory interface 104 to transport substrates between variouspositions/locations within factory interface 104 and to other locationsin communication therewith. Robots 108A-B may be configured to travelalong a track system within enclosure 104 from a first end to a secondend of the factory interface 104.

The processing system 100 may further include an integrated metrologychamber 117 to provide control signals, which may provide adaptivecontrol over any of the processes being performed in the processingchambers. The integrated metrology chamber 117 may include any of avariety of metrological devices to measure various film properties, suchas thickness, roughness, composition, and the metrology devices mayfurther be capable of characterizing grating parameters such as criticaldimensions, sidewall angle, and feature height under vacuum in anautomated manner.

Turning now to FIG. 2 is shown a cross-sectional view of an exemplaryprocess chamber system 200 according to the present technology. Chamber200 may be used, for example, in one or more of the processing chambersections 114 of the system 100 previously discussed Generally, the etchchamber 200 may include a first capacitively-coupled plasma source toimplement an ion milling operation and a second capacitively-coupledplasma source to implement an etching operation and to implement anoptional deposition operation. The ion milling operation may also becalled a modification operation. The chamber 200 may include groundedchamber walls 240 surrounding a chuck 250. In embodiments, the chuck 250may be an electrostatic chuck that clamps the substrate 202 to a topsurface of the chuck 250 during processing, though other clampingmechanisms as would be known may also be utilized. The chuck 250 mayinclude an embedded heat exchanger coil 217. In the exemplaryembodiment, the heat exchanger coil 217 includes one or more heattransfer fluid channels through which heat transfer fluid, such as anethylene glycol/water mix, may be passed to control the temperature ofthe chuck 250 and ultimately the temperature of the substrate 202.

The chuck 250 may include a mesh 249 coupled to a high voltage DC supply248 so that the mesh 249 may carry a DC bias potential to implement theelectrostatic clamping of the substrate 202. The chuck 250 may becoupled with a first RF power source and in one such embodiment, themesh 249 may be coupled with the first RF power source so that both theDC voltage offset and the RF voltage potentials are coupled across athin dielectric layer on the top surface of the chuck 250. In theillustrative embodiment, the first RF power source may include a firstand second RF generator 252, 253. The RF generators 252, 253 may operateat any industrially utilized frequency, however in the exemplaryembodiment the RF generator 252 may operate at 60 MHz to provideadvantageous directionality. Where a second RF generator 253 is alsoprovided, the exemplary frequency may be 2 MHz.

With the chuck 250 to be RF powered, an RF return path may be providedby a first showerhead 225. The first showerhead 225 may be disposedabove the chuck to distribute a first feed gas into a first chamberregion 284 defined by the first showerhead 225 and the chamber wall 240.As such, the chuck 250 and the first showerhead 225 form a first RFcoupled electrode pair to capacitively energize a first plasma 270 of afirst feed gas within a first chamber region 284. A DC plasma bias, orRF bias, resulting from capacitive coupling of the RF powered chuck maygenerate an ion flux from the first plasma 270 to the substrate 202,e.g., Ar ions where the first feed gas is Ar, to provide an ion millingplasma. The first showerhead 225 may be grounded or alternately coupledwith an RF source 228 having one or more generators operable at afrequency other than that of the chuck 250, e.g., 13.56 MHz or 60 MHz.In the illustrated embodiment the first showerhead 225 may be selectablycoupled to ground or the RF source 228 through the relay 227 which maybe automatically controlled during the etch process, for example by acontroller (not shown). In disclosed embodiments, chamber 200 may notinclude showerhead 225 or dielectric spacer 220, and may instead includeonly baffle 215 and showerhead 210 described further below.

As further illustrated in the figure, the etch chamber 200 may include apump stack capable of high throughput at low process pressures. Inembodiments, at least one turbo molecular pump 265, 266 may be coupledwith the first chamber region 284 through one or more gate valves 260and disposed below the chuck 250, opposite the first showerhead 225. Theturbo molecular pumps 265, 266 may be any commercially available pumpshaving suitable throughput and more particularly may be sizedappropriately to maintain process pressures below or about 10 mTorr orbelow or about 5 mTorr at the desired flow rate of the first feed gas,e.g., 50 to 500 sccm of Ar where argon is the first feedgas. In theembodiment illustrated, the chuck 250 may form part of a pedestal whichis centered between the two turbo pumps 265 and 266, however inalternate configurations chuck 250 may be on a pedestal cantileveredfrom the chamber wall 240 with a single turbo molecular pump having acenter aligned with a center of the chuck 250.

Disposed above the first showerhead 225 may be a second showerhead 210.In one embodiment, during processing, the first feed gas source, forexample, Argon delivered from gas distribution system 290 may be coupledwith a gas inlet 276, and the first feed gas flowed through a pluralityof apertures 280 extending through second showerhead 210, into thesecond chamber region 281, and through a plurality of apertures 282extending through the first showerhead 225 into the first chamber region284. An additional flow distributor or baffle 215 having apertures 278may further distribute a first feed gas flow 216 across the diameter ofthe etch chamber 200 through a distribution region 218. In an alternateembodiment, the first feed gas may be flowed directly into the firstchamber region 284 via apertures 283 which are isolated from the secondchamber region 281 as denoted by dashed line 223.

Chamber 200 may additionally be reconfigured from the state illustratedto perform an etching operation. A secondary electrode 205 may bedisposed above the first showerhead 225 with a second chamber region 281there between. The secondary electrode 205 may further form a lid or topplate of the etch chamber 200. The secondary electrode 205 and the firstshowerhead 225 may be electrically isolated by a dielectric ring 220 andform a second RF coupled electrode pair to capacitively discharge asecond plasma 292 of a second feed gas within the second chamber region281. Advantageously, the second plasma 292 may not provide a significantRF bias potential on the chuck 250. At least one electrode of the secondRF coupled electrode pair may be coupled with an RF source forenergizing an etching plasma. The secondary electrode 205 may beelectrically coupled with the second showerhead 210. In an exemplaryembodiment, the first showerhead 225 may be coupled with a ground planeor floating and may be coupled to ground through a relay 227 allowingthe first showerhead 225 to also be powered by the RF power source 228during the ion milling mode of operation. Where the first showerhead 225is grounded, an RF power source 208, having one or more RF generatorsoperating at 13.56 MHz or 60 MHz, for example, may be coupled with thesecondary electrode 205 through a relay 207 which may allow thesecondary electrode 205 to also be grounded during other operationalmodes, such as during an ion milling operation, although the secondaryelectrode 205 may also be left floating if the first showerhead 225 ispowered.

A second feed gas source, such as nitrogen trifluoride, and a hydrogensource, such as ammonia, may be delivered from gas distribution system290, and coupled with the gas inlet 276 such as via dashed line 224. Inthis mode, the second feed gas may flow through the second showerhead210 and may be energized in the second chamber region 281. Reactivespecies may then pass into the first chamber region 284 to react withthe substrate 202. As further illustrated, for embodiments where thefirst showerhead 225 is a multi-channel showerhead, one or more feedgases may be provided to react with the reactive species generated bythe second plasma 292. In one such embodiment, a water source may becoupled with the plurality of apertures 283. Additional configurationsmay also be based on the general illustration provided, but with variouscomponents reconfigured. For example, flow distributor or baffle 215 maybe a plate similar to the second showerhead 210, and may be positionedbetween the secondary electrode 205 and the second showerhead 210. Asany of these plates may operate as an electrode in variousconfigurations for producing plasma, one or more annular or other shapedspacer may be positioned between one or more of these components,similar to dielectric ring 220. Second showerhead 210 may also operateas an ion suppression plate in embodiments, and may be configured toreduce, limit, or suppress the flow of ionic species through the secondshowerhead 210, while still allowing the flow of neutral and radicalspecies. One or more additional showerheads or distributors may beincluded in the chamber between first showerhead 225 and chuck 250. Sucha showerhead may take the shape or structure of any of the distributionplates or structures previously described. Also, in embodiments a remoteplasma unit (not shown) may be coupled with the gas inlet to provideplasma effluents to the chamber for use in various processes.

In an embodiment, the chuck 250 may be movable along the distance H2 ina direction normal to the first showerhead 225. The chuck 250 may be onan actuated mechanism surrounded by a bellows 255, or the like, to allowthe chuck 250 to move closer to or farther from the first showerhead 225as a means of controlling heat transfer between the chuck 250 and thefirst showerhead 225, which may be at an elevated temperature of 80°C.-150° C., or more. As such, an etch process may be implemented bymoving the chuck 250 between first and second predetermined positionsrelative to the first showerhead 225. Alternatively, the chuck 250 mayinclude a lifter 251 to elevate the substrate 202 off a top surface ofthe chuck 250 by distance H1 to control heating by the first showerhead225 during the etch process. In other embodiments, where the etchprocess is performed at a fixed temperature such as about 90-110° C. forexample, chuck displacement mechanisms may be avoided. A systemcontroller (not shown) may alternately energize the first and secondplasmas 270 and 292 during the etching process by alternately poweringthe first and second RF coupled electrode pairs automatically.

The chamber 200 may also be reconfigured to perform a depositionoperation. A plasma 292 may be generated in the second chamber region281 by an RF discharge which may be implemented in any of the mannersdescribed for the second plasma 292. Where the first showerhead 225 ispowered to generate the plasma 292 during a deposition, the firstshowerhead 225 may be isolated from a grounded chamber wall 240 by adielectric spacer 230 so as to be electrically floating relative to thechamber wall. In the exemplary embodiment, an oxidizer feed gas source,such as molecular oxygen, may be delivered from gas distribution system290, and coupled with the gas inlet 276. In embodiments where the firstshowerhead 225 is a multi-channel showerhead, any silicon-containingprecursor, such as OMCTS for example, may be delivered from gasdistribution system 290, and directed into the first chamber region 284to react with reactive species passing through the first showerhead 225from the plasma 292. Alternatively the silicon-containing precursor mayalso be flowed through the gas inlet 276 along with the oxidizer.Chamber 200 is included as a general chamber configuration that may beutilized for various operations discussed in reference to the presenttechnology. The chamber is not to be considered limiting to thetechnology, but instead to aid in understanding of the processesdescribed. Several other chambers known in the art or being developedmay be utilized with the present technology including any chamberproduced by Applied Materials Inc. of Santa Clara, Calif., or anychamber that may perform the techniques described in more detail below.

FIG. 3 illustrates an etching method 300 that may be performed, forexample, in the chamber 200 as previously described. The method may alsobe performed in any other chamber in which a substrate-level plasma maybe formed as well as a plasma within a remote plasma region may beformed. Method 300 may include one or more operations prior to theinitiation of the method, including front end processing, deposition,etching, polishing, cleaning, or any other operations that may beperformed prior to the described operations. A processed substrate,which may be a semiconductor wafer of any size, may be positioned withina chamber for the method 300. In embodiments the operations of method300 may be performed in multiple chambers depending on the operationsbeing performed. Additionally, in embodiments the entire method 300 maybe performed in a single chamber to reduce queue times, contaminationissues, and vacuum break. Subsequent operations to those discussed withrespect to method 300 may also be performed in the same chamber or indifferent chambers as would be readily appreciated by the skilledartisan.

Method 300 may include forming an inert plasma within a remote plasmaregion of a semiconductor processing chamber at operation 305. Theremote plasma may be a first plasma in embodiments. A substrate mayalready be positioned within the chamber prior to operation 305. Withreference to chamber 200 for illustration purposes only, the plasma maybe formed or generated in region 292, or within a region separated fromthe processing region in which the substrate resides. The separation maybe with an electrode, allowing containment of the plasma formed. Method300 may also include forming a plasma at the substrate level within theprocessing region of the chamber at operation 310, or within a regiondefined at least in part by the substrate support pedestal. Such aplasma is similarly understood to be a local plasma or wafer-levelplasma, and may be a second plasma in embodiments of the presenttechnology.

In some embodiments, both the remote plasma and the wafer-level plasmamay be capacitively-coupled plasmas. In some embodiments the wafer-levelplasma may be a bias plasma formed within the processing region of thechamber. The wafer-level plasma may be formed from the same inertprecursor as the remote plasma, and may include plasma effluents thatmay flow from the remote plasma region into the processing region. Thesecond plasma region may be used to produce ions or other effluents ofthe inert plasma in method 300 for modifying a surface of an exposedmaterial on a semiconductor substrate at operation 315. Accordingly,plasma may be formed in both the remote plasma region and the processingregion during some embodiments of method 300.

Subsequent the surface modification of the exposed material on thesubstrate surface, the second plasma, or the wafer-level plasma, may beextinguished at operation 320. While the second plasma may beextinguished, the first plasma or remote plasma may be maintained.Maintaining the remote plasma may be performed by maintaining a flow ofan inert precursor throughout the method, which may allow the remoteplasma to be sustained. Ions developed in the remote plasma may befiltered by an ion suppressor, as will be discussed below. As explainedpreviously, by maintaining the first plasma, effects of cycling thestriking of the first plasma may be reduced or eliminated.

Method 300 may also include adding an etchant precursor, which mayinvolve flowing an etchant precursor to the remote plasma region atoperation 325, while the remote plasma is maintained. Maintaining thefirst plasma may include a controlled switching of precursor delivery,as well as a compensating effect in which the inert precursor may bereduced or adjusted while the etchant precursor may be added. The remoteplasma may produce etchant plasma effluents, and the etchant precursormay include or be composed of a fluorine-containing precursor. Inembodiments, the plasma utilized in operation 325 may also be formed atthe wafer level, but a remote plasma may reduce a sputtering componentat the wafer and from the chamber components. The etchant plasmaeffluents may be flowed through the processing chamber to the processingregion of the semiconductor processing chamber where the substrate ishoused at operation 330. Upon contacting the modified surface, theetchant plasma effluents may remove the modified surface of the exposedmaterial from the semiconductor substrate at operation 335.

The modifying and removal operations of method 300 may allow acontrolled removal of unwanted materials, which may include oxides,nitrides, or other materials from the substrate. The operations may alsobe well suited for any size features, including small pitch features, orwhere the width between successive features for example, is less than orabout 50 nm, less than or about 25 nm, less than or about 20 nm, lessthan or about 15 nm, less than or about 12 nm, less than or about 10 nm,less than or about 9 nm, less than or about 8 nm, less than or about 7nm, less than or about 6 nm, less than or about 5 nm, less than or about4 nm, less than or about 3 nm, less than or about 2 nm, less than orabout 1 nm, or smaller. The modifying and removal operations may beperformed successively in multiple chambers or in a single chamber, suchas, for example, chamber 200, that may produce both wafer-level plasmasand remote plasmas within the chamber, or in association with thechamber.

The modifying operation 315 may involve an inert plasma of one or morematerials. The material used to produce the plasma may be one or morenoble materials including helium, neon, argon, krypton, xenon, or radon.The material used to produce the plasma may also be additional materialsthat may have limited chemical activity or be unreactive with theexposed material on the semiconductor surface being modified. Forexample, hydrogen may be used in operations 305-315, and in embodimentsthe inert plasma may either comprise or consist of a hydrogen plasma ora helium plasma. The hydrogen plasma may be generated from any number ofhydrogen containing materials or mixtures, and may be formed exclusivelyof hydrogen (H₂) in embodiments. The modifying operation may involve aform of bombardment of the material to be removed. With hydrogen being asmall, light material, it may be less likely to sputter the material atwhich it is being directed than heavier materials such as, for example,helium. In other embodiments, helium may be used to allow modificationof stronger bonding structures, which may be broken by heavier ions.

The plasma formed from the inert precursor may be a bias plasmaproviding directional flow of plasma effluents to the substrate. Theplasma may be a low-level plasma to limit the amount of bombardment,sputtering, and surface modification. In embodiments the plasma powermay be less than or about 300 W, less than or about 250 W, less than orabout 200 W, less than or about 150 W, less than or about 100 W, lessthan or about 75 W, less than or about 50 W, or less than or about 25 W.By utilizing a plasma power that is, for example, about 50 W, the depthof penetration of the plasma effluents may be limited. For example,modification operations as described, may allow the surface of theexposed material on the semiconductor substrate to be modified to adepth from the exposed surface within the semiconductor substrate ofless than or about 4 nm, and may allow modification of the surface ofmaterials to a depth of less than or about 3 nm, less than or about 2nm, or less than or about 1 nm. For example, by utilizing the low-levelplasma, such as at about 50 W, and a relatively light precursor such ashydrogen, the saturation depth of penetration may be around 1 nm inembodiments. The modification operation may be relatively or completelyinsensitive to temperature and material, and may modify or damageexposed regions of nitride, oxide, or polysilicon almost equally, andmay also modify metal-containing materials similarly.

The pressure within the processing chamber may be controlled during themethod 300 as well. For example, while forming the inert plasma andperforming the modification operation, the pressure within theprocessing chamber may be maintained below or about 1 Torr.Additionally, in embodiments, the pressure within the processing chambermay be maintained below or about 500 mTorr, below or about 250 mTorr,below or about 200 mTorr, below or about 150 mTorr, below or about 100mTorr, below or about 80 mTorr, below or about 60 mTorr, below or about50 mTorr, below or about 40 mTorr, below or about 30 mTorr, below orabout 20 mTorr, below or about 10 mTorr, or lower. The pressure withinthe chamber may affect the directionality of the modification operation315. For example, as pressure is increased, the modification process maybecome more isotropic, and as the pressure is reduced, the modificationprocess may become more anisotropic. Thus, as pressure is increased,vertical structure sidewalls may begin to be treated as well, whichafter removal can remove material beyond what was originally desired incertain operations. Accordingly, in embodiments the pressure may bemaintained between about 10 mTorr and about 150 mTorr, for example, tomaintain a relatively or substantially anisotropic profile of themodification operation.

The plasma utilized in the removal operation may be formed remotely fromthe processing region of the semiconductor processing chamber. Forexample, the plasma may be formed in a region of the semiconductorprocessing chamber that is fluidly isolated from the processing regionof the semiconductor processing chamber. Thus, the region may bephysically separated from the processing region, while being fluidlycoupled with the processing region. For example, in the exemplarychamber of FIG. 2, the remote plasma may be generated in region 292,which is separated from the processing region by showerhead 225.Additionally, the remote plasma may be formed in a remote plasma unit,such as an RPS unit that is separate from the chamber, but fluidlycoupled with the chamber to deliver plasma effluents into the chamber,such as through a lid, top plate, or showerhead.

The remote plasma may be formed from one or more etchant precursorsincluding a fluorine-containing precursor. The fluorine-containingprecursor may include one or more materials including NF₃, HF, F₂, CF₄,CHF₃, C₂F₆, C₃F₆, BrF₃, ClF₃, SF₆, or additional fluorine-substitutedhydrocarbons, or fluorine-containing materials. In embodiments theetchant precursor may also be or include a nitrogen-containingprecursor. The fluorine-containing precursor may be flowed into theremote plasma region to generate plasma effluents, such asfluorine-containing plasma effluents. A source of hydrogen may also beincorporated as an etchant precursor, and may include hydrogen, ammonia,or any other incompletely substituted hydrocarbon, or otherhydrogen-containing material. Sources of oxygen may also be utilized insome etching operations, which may include oxygen, ozone,nitrogen-and-oxygen-containing materials, oxygen-and-hydrogen-containingmaterials, or other fluids including oxygen. In some embodiments, one ormore precursors may bypass the remote plasma region and be deliveredinto the processing region where the precursor may interact with thefluorine-containing plasma effluents. In either scenario, the plasmaeffluents may be delivered to the processing region of the semiconductorprocessing chamber where they may contact or interact with the modifiedmaterial on the semiconductor substrate.

As previously discussed, forming a plasma in a region in which theelectrodes are closer, may conventionally require increasing thepressure within the processing chamber. Although in some embodiments thepressure within the processing chamber may be increased during theetching operations, in some embodiments the pressure may be maintainedin any of the ranges described above, such as below or about 100 mTorr.The present technology may differ from other conventional designs toallow formation or maintenance of plasma at these pressures within asmaller gap region of the processing chamber. While some conventionaltechnologies would be incapable of maintaining plasma at the notedpressures when the plasma is generated at a frequency of 13.56 MHz, thepresent technology may form the plasma at higher frequency to allowformation at lower pressure.

In some embodiments, the present technology may form the remote plasmaat operation 305 at a frequency above or about 30 MHz, and may form theremote plasma at a frequency of above or about 40 MHz, above or about 50MHz, above or about 60 MHz, above or about 70 MHz, above or about 80MHz, above or about 90 MHz, above or about 100 MHz, or higher. In someembodiments the plasma may be formed between about 40 MHz and about 80MHz. Frequencies below about 40 MHz may be incapable of maintaining aplasma at pressures of less than 1 Torr, or less than 500 mTorr, or lessthan 200 mTorr. Additionally, at higher frequencies, current maytransfer and cause issues with other components. For example, as thefrequency increases above 60 MHz, or above 80 MHz, current generated maymore easily flow to other lines in the system, which can damagesubsystem power supplies or cause other issues.

Increasing the frequency at which the plasma is generated in the remoteregion may also provide additional benefits. For example, theoretically,forming the plasma at lower pressures is likely to reduce the radicalconcentration of the etchants. However, increasing the frequency toabove or about 40 MHz, for example, may compensate for the reduction andmay increase the radical concentration within the plasma effluentsformed. Additionally, increasing the frequency may allow thedissociation to be performed at lower power. For example, to create acertain plasma profile at a frequency of 13.56 MHz, the plasma may begenerated at a power of from about 300 W to about 1 kW or more. However,when the frequency is above or about 40 MHz, the same profile may beproduced at a power of from about 80 W to about 200 W, or less.

As previously discussed, the precursors and plasma effluents may beeffective at removing oxide and/or nitride, or other previously notedmaterials, in various semiconductor processes. Selective removal,however, may be affected by processing temperatures. Lower chambertemperatures may allow increased etching of one or more materials. Inprocesses utilizing a fluorine-containing precursor and also ahydrogen-containing precursor to etch oxide materials, for example only,the process may involve performing an etch with plasma effluents, orwith alternative precursors such as HF, at a low temperature, such asbelow about 50° C. or lower, to form solid byproducts on the surface ofthe material being removed. The procedure may then involve heating thematerials above around 100° C. in order to sublimate the solidbyproducts, which may include ammonium fluorosilicate, for example.

The present technology, however, may perform the removal processes at asemiconductor substrate or semiconductor chamber temperature of above orabout 50° C., above or about 60° C., above or about 70° C., above orabout 80° C., above or about 90° C., above or about 100° C., above orabout 110° C., above or about 120° C., above or about 130° C., above orabout 140° C., or above or about 150° C. An etching process utilizingthe precursors discussed above may have limited capability, or may notetch certain materials at all at a temperature of about 100° C., forexample. While conventional technologies may avoid such temperatures asthey may prevent the desired removal, the present technology can utilizethis benefit to provide a self-limiting stop on the etching operation.Although unmodified oxide and nitride materials may not etch with theremoval process described at a temperature of 100° C., the modifiedmaterials produced may etch at a sufficient rate to remove the unwantedmaterials.

Thus, once the modified portion of the exposed materials has beenremoved, the underlying unmodified materials may not etch, or may havelimited etching, and may effectively halt the etching process. In thisway, minute amounts of material may be removed without overly attackingthin semiconductor layers or small pitch features. Accordingly, inembodiments, removing the modified surface of the exposed material mayexpose an unmodified portion of the material. An etching selectivity ofa modified portion of the material to an unmodified portion of thematerial may be greater than or about 10:1. Depending on the materialbeing etched, an etching selectivity of a modified portion of thematerial to an unmodified portion of the material may be greater than orabout 20:1, 40:1, 100:1, 1,000:1, 10,000:1, up to about 1:0 at whichpoint the modified portion of the material etches, but an unmodifiedportion of the material does not etch. The modification operation mayproduce an amount of dangling bonds and reactive sites for the modifiedmaterial, which may allow the removal operation to occur underconditions at which the removal may not otherwise occur, or may occur atsubstantially reduced rates and selectivities for unmodified materials.

As previously explained, the modification operations may be performed ata relatively low plasma power level to create a depth of penetrationwithin the exposed material surfaces of a few nanometers or less, suchas about 1 nm to about 2 nm. Because the removal operation can belimited to essentially only remove modified surfaces, or have limitedimpact on unmodified surfaces, the removal operation may be limited tothe modified region, and thus remove about 1 nm, about 2 nm, about 3 nm,or about 4 nm of material. The modification operation may have asaturation depth of about 1 nm in embodiments, but an amount ofmodification or penetration may occur to up to 2 nm, up to 3 nm, or upto 4 nm, although the saturation depth may be much less. However, theremoval operation may continue to etch partially modified regions ofmaterial, and thus the removal operation may remove slightly morematerial than the saturation depth of the modification.

To ensure removal of all unwanted material from a substrate, themodification and removal operations may be performed in cycles to allowremoval to a depth beyond the typical saturation depth of themodification operation. Accordingly, in embodiments, method 300 may beperformed for 1 cycle, 3 cycles, 5 cycles, 10 cycles, 30 cycles, 50cycles, or more in order to fully remove a material from a substrate.For various removals, the fine-tune control over the material removalbased on a saturation depth of the modification operation may allowabout 1 nm, about 2 nm, about 3 nm, or about 4 nm to be removed eachcycle. The cycle may not include all operations of method 300. Forexample, after the material removal, the method may include halting aflow of the etchant precursor while maintaining the remote plasma. Themethod may then return to operation 310, and generate the second plasma,which may be a bias plasma, and modify an additional amount of theexposed material. The other operations of method 300 may similarly berepeated. Across all cycles, the remote plasma may be maintained.Accordingly, by cycling the wafer-level plasma and the introduction ofthe etchant precursors, the present technology may provide improvedetching processes.

In this way, within 1 cycle, within 2 cycles, within 3 cycles, or withinabout 4 cycles the entire material may be removed from the substrate ata total removal after all cycles of less than or about 20 nm, less thanor about 15 nm, less than or about 12 nm, less than or about 11 nm, lessthan or about 10 nm, less than or about 9 nm, less than or about 8 nm,less than or about 7 nm, less than or about 6 nm, less than or about 5nm, less than or about 4 nm, less than or about 3 nm, less than or about2 nm, or less than or about 1 nm. The operations are being discussedwith respect to a limited amount of removal, but the techniques can alsobe used to remove additional material by, for example, causing themodification to occur to a lower depth, increasing the number of cycles,or by adjusting etching parameters including temperature. However, forlimiting the amount of removal in many semiconductor processingoperations, the low-power bias plasma with precursors such as previouslydiscussed may allow limited material to be removed with each cycle.

Turning to FIGS. 4A-4B is illustrated schematic cross-sectional views ofan exemplary semiconductor processing chamber 400 in which operations ofthe present technology are being performed. As shown in FIG. 4A, chamber400 may include a faceplate 405 through which inert precursors and/oretchant precursors may be distributed. Chamber 400 may include a lid403, which may facilitate precursor distribution. Although a singleinlet is illustrated for chamber 400, additional inlets may be includedto allow introduction of multiple precursors in different patterns.Chamber 400 may include an ion suppressor 410, which may allow filteringof generated ions. In some embodiments, as illustrated, faceplate 405and ion suppressor 410 may operate as electrodes to generate a plasmawithin a first plasma region 415. An RF source 417 may be coupled withfaceplate 405, while ion suppressor 419 may be grounded in embodiments.Accordingly, an insert 407, such as a dielectric material, may bepositioned between the faceplate 405 and ion suppressor 410 to allowformation of a plasma to occur.

Chamber 400 may or may not include a showerhead 425 in embodiments.Showerhead 425 may allow introduction of additional precursors, and mayalso aid in uniform distribution of precursors through the processingchamber 400. Showerhead 425 may also partially define a second plasmaregion 435 with substrate support 430. For example, a bias plasma may beformed between showerhead 425 and substrate support 430 on which asubstrate 440 may be positioned. During operation, a plasma may bestruck in both the first plasma region 415 and the second plasma region435 from an inert precursor 401 delivered into the chamber, or aprecursor that may not chemically react, or may have limited chemicalreaction with an exposed material on substrate 440. Although ions formedin first plasma region 415 may be filtered by ion suppressor 410, ions445 may be developed in second plasma region 435. Ions 445 may bedeveloped to perform the modification of the exposed materials aspreviously described.

As discussed above, first plasma region 415 may be characterized by afirst gap distance defined between the electrodes, which may befaceplate 405 and ion suppressor 410. Additionally, second plasma region435 may be characterized by a second gap distance defined between theelectrodes, which may be showerhead 425 and substrate support 430. Insome embodiments, the first gap distance may be less than the second gapdistance. Accordingly, to produce plasma within the region at similaroperating pressures, first plasma region 415 may be coupled with asource that provides power at a frequency of at least about 40 MHz toallow formation at pressures described previously. Thus, plasma may beformed in both regions simultaneously during methods of the presenttechnology.

Subsequent a first period of time during which the modificationoperation is performed, plasma within second plasma region 435 may beextinguished, while plasma is maintained in first plasma region 415, asillustrated in FIG. 4B. An etchant precursor 402 may be delivered intoprocessing chamber 400 and into first plasma region 415, along with theinert precursor 401 in some embodiments, which may allow the plasma tobe maintained in the first plasma region 415. Because ions may befiltered from plasma effluents by ion suppressor 410, the inertprecursor may continue to flow without further modifying materials onsubstrate 440.

Etchant plasma effluents may be flowed from first plasma region 415,without further enhancement in second plasma region 435. The plasmaeffluents may contact the modified material on substrate 440, and mayremove the materials from the substrate. In some embodiments, theoperations may be repeated to modify and remove additional material fromthe substrate. During each cycle, the first plasma may be maintainedwithin the remote plasma region. The modification and etching operationsmay be performed for any length of time, which may be similar ordifferent in methods encompassed by the present technology. For example,either or both operations may be performed for a period of time greaterthan or about 1 second, greater than or about 5 seconds, greater than orabout 10 seconds, greater than or about 20 seconds, greater than orabout 30 seconds, greater than or about 40 seconds, greater than orabout 50 seconds, greater than or about 60 seconds, or more. Bymaintaining the plasma in the first plasma region 415 throughout theoperations performed, many of the issues described elsewhere can beavoided, while providing the discussed benefits.

In the preceding description, for the purposes of explanation, numerousdetails have been set forth in order to provide an understanding ofvarious embodiments of the present technology. It will be apparent toone skilled in the art, however, that certain embodiments may bepracticed without some of these details, or with additional details.

Having disclosed several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theembodiments. Additionally, a number of well-known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent technology. Accordingly, the above description should not betaken as limiting the scope of the technology.

Where a range of values is provided, it is understood that eachintervening value, to the smallest fraction of the unit of the lowerlimit, unless the context clearly dictates otherwise, between the upperand lower limits of that range is also specifically disclosed. Anynarrower range between any stated values or unstated intervening valuesin a stated range and any other stated or intervening value in thatstated range is encompassed. The upper and lower limits of those smallerranges may independently be included or excluded in the range, and eachrange where either, neither, or both limits are included in the smallerranges is also encompassed within the technology, subject to anyspecifically excluded limit in the stated range. Where the stated rangeincludes one or both of the limits, ranges excluding either or both ofthose included limits are also included.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “a layer” includes aplurality of such layers, and reference to “the precursor” includesreference to one or more precursors and equivalents thereof known tothose skilled in the art, and so forth.

Also, the words “comprise(s)”, “comprising”, “contain(s)”, “containing”,“include(s)”, and “including”, when used in this specification and inthe following claims, are intended to specify the presence of statedfeatures, integers, components, or operations, but they do not precludethe presence or addition of one or more other features, integers,components, operations, acts, or groups.

1. An etching method comprising: forming a remote plasma of an inertprecursor in a remote plasma region of a processing chamber; forming abias plasma of the inert precursor within a processing region of theprocessing chamber; modifying a surface of an exposed material on asemiconductor substrate within the processing region of the processingchamber with plasma effluents of the inert precursor; extinguishing thebias plasma while maintaining the remote plasma; adding an etchantprecursor to the remote plasma region to produce etchant plasmaeffluents; flowing the etchant plasma effluents to the processing regionof the processing chamber; and removing the modified surface of theexposed material from the semiconductor substrate.
 2. The etching methodof claim 1, wherein the inert precursor comprises hydrogen or helium. 3.The etching method of claim 1, wherein the remote plasma regioncomprises a region within the processing chamber separated from theprocessing region by a showerhead.
 4. The etching method of claim 3,wherein the remote plasma region is characterized by a smaller gapbetween electrodes within the processing chamber than the processingregion.
 5. The etching method of claim 1, wherein the method isperformed at a chamber operating pressure below about 500 mTorr.
 6. Theetching method of claim 1, wherein the etchant precursor comprises afluorine-containing precursor or a nitrogen-containing precursor.
 7. Theetching method of claim 1, wherein the remote plasma comprises acapacitively-coupled plasma.
 8. The etching method of claim 7, whereinthe remote plasma is formed at an electrical frequency of greater thanor about 40 MHz.
 9. The etching method of claim 8, wherein the remoteplasma is formed at an electrical frequency less than or about 80 MHz.10. The etching method of claim 1, further comprising: halting a flow ofthe etchant precursor while maintaining the remote plasma; forming abias plasma in the processing region; and modifying an additional amountof the exposed material.
 11. The etching method of claim 10, whereinflowing and halting the flow of the etchant precursor is performed for aplurality of cycles.
 12. The etching method of claim 11, wherein theremote plasma is maintained throughout the plurality of cycles.
 13. Anetching method comprising: forming a first plasma within a remote plasmaregion of a processing chamber; forming a second plasma within aprocessing region of the processing chamber; modifying an exposedmaterial on a semiconductor substrate within the processing region ofthe processing chamber with effluents of the second plasma;extinguishing the second plasma while maintaining the first plasma;providing an etchant precursor to the remote plasma region to formetchant plasma effluents; and etching the modified exposed material onthe semiconductor substrate.
 14. The etching method of claim 13, whereinthe etching is performed at a temperature of about 100° C.
 15. Theetching method of claim 13, wherein the remote plasma region of theprocessing chamber is fluidly coupled with, and physically separatedfrom, the processing region of the processing chamber.
 16. The etchingmethod of claim 15, wherein the first plasma is a capacitively-coupledplasma operated at a power level of about 500 W or less.
 17. The etchingmethod of claim 15, wherein the first plasma is a capacitively-coupledplasma operated at a frequency of about 40 MHz or more.
 18. The etchingmethod of claim 13, further comprising: halting a flow of the etchantprecursor while maintaining the first plasma; reforming the secondplasma in the processing region of the processing chamber; and modifyingan additional amount of the exposed material on the semiconductorsubstrate.
 19. An etching method comprising: striking a plasma of aninert precursor in a remote plasma region of a processing chamber,wherein the remote plasma region is characterized by a first gap betweenelectrodes within the processing chamber; striking a plasma of the inertprecursor within a processing region of the processing chamber, whereinthe processing region of the processing chamber is characterized by asecond gap between electrodes within the processing chamber, and whereinthe second gap between electrodes is greater than the first gap betweenelectrodes; modifying a surface of a semiconductor substrate within theprocessing region of the processing chamber with plasma effluents of theinert precursor; extinguishing the plasma within the processing regionof the processing chamber while maintaining the plasma in the remoteplasma region; flowing an etchant precursor to the remote plasma regionto produce etchant plasma effluents; flowing the etchant plasmaeffluents to the processing region of the processing chamber; andremoving the modified surface of the semiconductor substrate.
 20. Theetching method of claim 19, wherein the plasma formed in the remoteplasma region is formed at a frequency of at least about 40 MHz.